Line a cookie sheet with a macaron silicone mat or parchment paper with 1 ½ inch circles drawn onto the paper.
Prepare macarons. Sift powdered sugar, almond flour, and instant espresso powder two times through a fine sieve.
Place egg whites in bowl of a stand mixer and whip with the whisk attachment until soft peaks form. Gradually add sugar while the mixer is running and continue to whip until stiff peaks form. Add a third of the sifted dry ingredients to the egg whites, folding them into the egg whites with a spatula. Add remaining dry ingredients in two more batches. Continue to gently fold batter until ingredients are fully combined. The batter is ready when you can hold the spatula over the bowl and draw a figure 8 smoothly without the batter breaking off (about 50-60 folds).
Transfer batter to a piping bag and pipe 1 ½ inch discs onto the prepared cookie sheet. Firmly tap the cookie sheet on the counter 3 or 4 times to bring any air bubbles to the surface. If there are any remaining bubbles, pop them with a toothpick.
Let macarons sit at room temperature until a skin forms on the top, about 1 hour.
Preheat oven to 280 degrees F.
Bake macarons in the center rack of the oven for 15 minutes, rotating the pan halfway through the baking time. Remove from oven and cool completely on the baking sheet.
While macarons are cooling, make caramel buttercream. Beat butter in the bowl of a stand mixer until softened, about 1 minute. Add caramel and vanilla and beat for another minute or until well combined. Add powdered sugar and the pinch of salt and continue to beat until smooth, about 2 minutes. Transfer buttercream to a piping bag.
Peel macarons off the baking sheet. Flip half of the macarons over and pipe buttercream on top. Sandwich with another cookie and serve.